A pneumatic tire has two inextensible beads which are retained on a wheel rim by radially projecting bead flanges at each side of the rim and rest on the base of the rim between the flanges. A single piece rim is commonly used, because it can be mass-produced cheaply and for tubeless tires only requires to be sealed at the valve hole. In order to make it possible for the beads to pass over the bead flanges, a well is provided in the base of the rim, into which one side of both beads can be sunk so that their opposite sides are raised high enough to pass over the bead flange. After the tire is fitted, the well performs no useful function until it is necessary to remove the tire. This done by a reverse operation, the beads being dislodged into the well.
In normal service the well of the rim presents no problem, but if a tire failure such as a blowout occurs, it presents a real hazard particularly at the high speeds commonplace today which nowadays is considered to be an unacceptable risk. After a blowout, and loss of pressure, one, or both tire beads frequently become dislodged by the forces acting on the wheel and tire and find their way into the well. Once this happens, the tire bead is able, under the stresses present, to pass over the rim flange, and set up steering reactions that cannot be controlled by the driver.
If the tire beads could be prevented from going into the well after a blowout, the deflated tire will remain on the rim, and improved vehicle control is possible.
There are multi-part types of rim that take apart for tire servicing, adn therefore do not require a well, but these are costly, and difficult to seal for tubeless application. The most common type also have the disadvantage that they utilise the air pressure within the tire to maintain an integral assembly. When this pressure is lost, as in a blowout, the rim frequently falls apart, and sheds its tire.
There has recently been made public, in U.K. P.S. 1160412, a proposal for a deformable one piece rim. The diameter of one of the flanges of a well-less rim is increased, or else a partial well is eliminated, by deforming the rim in a press tool after the tire has been fitted.
We are approaching it from an entirely different direction, one which in our view has many advantages. It depends on providing a well in the rim but obstructing access to the well at times when such access, in the event of tire deflation, would be dangerous.
Partial or complete obstruction of the well of a rim has been proposed before, for entirely different purposes. There are many early patents where inventors have been concerned to shift a well right to one side of the rim in order to allow as much room as possible within the radius of the rim for brake drums. Because they had done this, they had to provide something on which the bead of the tubed tire could be supported. See, for example, U.K. Pat. No. 304297 and 248305. It was also common to wrap a fabric layer or a split ring round the rim within the well of a spoked wheel to prevent the ends of the spokes from chafing the tube of a tubed tire on the rim (see U.K. Pat. No. 248305). This is still common practice with bicycle wheels today.
A modern example of the former type of proposal is seen in U.S. Pat. No. 2840133. Here the inventor was faced with the additional difficulty of how to inflate his tubeless tire when, by shifting the well right up next to a flange, he had deprived it of its usual initial seat during inflation (i.e. with the heel of the bead on the base of the rim at the flange). His invention was directed at how to inflate a tubeless tire in this situation, which he solves by having means retaining the tire bead against which the bead seals inboard of the well for initial inflation, and special air passages from a permanently bridged portion of the well into which an inlet inflation valve opens. When inflation pressure is high enough, the bead flips outwardly over the bridging strip, and seats hard against the flange of the rim. This same action wedges the frustoconical bridging strip into position. There is nothing, apart from the presence of the bead of the tire, holding the bridging strip in place. It should also be noted that the ends of the strip do not meet and are not connected together.